Despite their small height and stunted appearance, mangroves along the desert coasts of Baja California have compensated for sea-level rise during the last two millennia by accreting on their own root remains. In doing so, they have accumulated very large amounts of carbon in their sediments (900–3,000 Mg C/ha), often higher than that accumulated under tall, lush, tropical mangrove forests. Mangroves represent the largest carbon sink per unit area in Mexico’s northern drylands. Our results highlight the global importance of mangrove conservation in this region.